php require 在类的继承中的坑
对于php的文件调用,我们有时会为使用include, include_once, require, require_once而陷入斟酌,不仅初学者如此,高级工程是由也会有纠结的时候。而且至今使用哪一个为最优仍没有达成共识。每个团队每个项目有自己的代码规范,也有自己的使用理由。至于四个函数的异同,在此不作赘述。
我们团队推崇使用的是require_once,为什么使用require_once而不是require呢。今日给出一个理由。
require的使用在多个子类继承同一个父类时会引起redeclare的错误
因为require不是一个方法,而是一个语句块。所以,他把把引用的文件执行一遍,所以,如果你在一个文件里引用了两个继承自同一父类的类时,由会引起上述问题。而require_once则不会,它会先判断是否引用过,苦有则略过。
看代码,先写一个抽象父类:
[php]
AbstractClass.php
AbstractClass.php
再实现两个子类:
[php]
RequireClassA.php
//使用require而不是require_once
require 'AbstractClass.php';
class RequireClassA extends AbstractClass{
}
RequireClassA.php
//使用require而不是require_once
require 'AbstractClass.php';
class RequireClassA extends AbstractClass{
}
[php]
RequireClassB.php
//使用require而不是require_once
require 'AbstractClass.php';class RequireClassB extends AbstractClass{}
RequireClassB.php
//使用require而不是require_once
require 'AbstractClass.php';class RequireClassB extends AbstractClass{}
实现调用:
[php]
test.php
require_once 'RequireClassB.php';
require_once 'RequireClassA.php';
$A = new RequireClassA();
$B = new RequireClassB();
test.php
require_once 'RequireClassB.php';
require_once 'RequireClassA.php';
$A = new RequireClassA();
$B = new RequireClassB();
运行test.php报错
[plain]
>php test.php
>PHP Fatal error: Cannot redeclare class AbstractClass in /home/john/workspace/php/require/AbstractClass.php on line 2
>php test.php
>PHP Fatal error: Cannot redeclare class AbstractClass in /home/john/workspace/php/require/AbstractClass.php on line 2

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